The deaths occurred at Kenya's Maasai Mara National Reserve, as the herd was beginning its swing to the east on its way back to the Serengeti.
Since the drownings, the animals' bodies have washed downriver, beaching on the Mara's muddy banks and getting caught under a nearby bridge, Lemaire wrote on her blog for the nonprofit WildlifeDirect.
The remains formed what she described as "pungent islands of bloated carcasses."
"The crocodiles, storks, and vultures have not had to worry about where to find their next meal," she wrote.
"Those that aren't consumed will be left and will eventually decompose in the water. These thousands of carcasses will undoubtedly affect the health of the water, but to what extent, only time will tell."
Lemaire also declined to speculate, in her email to National Geographic News, on the impact the mass deaths might have on the wildebeests' overall population health.
"I would imagine that such a significant decrease in population would have an effect," she said, "but what that effect would be and to what extent, I cannot say."
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